Glare protection or dazzle protection devices are, for example, used in welding protection masks, helmets or goggles. In order to enhance the safety and productivity of the welder, active electro-optical cells or filter elements are used, which can be driven electronically to a bright and dark state respectively, without the need of mechanical movement. Electro-optical filter devices for glare protection typically comprise a liquid crystal cell or LC-cell which is controlled to block light transmission to a greater or lesser extent when a light sensor detects a light intensity exceeding a predefined threshold level and/or exhibiting certain dynamic properties such as jitter or flickering. Furthermore, electro-optical filter devices are known in the prior art which automatically adjust the filter transmission to the varying brightness conditions encountered in a welding situation.
U.S. Pat. No. 4,620,322 shows an electro-optic welding lens assembly in which a light sensing element for controlling the transmission of a LC filter doubles as a power supply for powering the control circuitry and the LC filter.
In U.S. Pat. No. 4,863,244, a welding lens assembly is disclosed which comprises a manually adjustable transmission with an additional automatic override circuit using a brightness sensor for measuring the amount of light falling on the lens assembly. The welder may set the transmission level manually, but if the amount of light exceeds a predetermined threshold, the transmission of light is automatically reduced, compensating for the excess light.
In U.S. Pat. No. 4,920,257, a light filter with the automatic regulation of transmission is described, in which a first optical sensor is arranged behind the filter element in the radiation direction, and a second sensor is arranged beside or in front of the filter element. A subtraction circuit determines the difference between the two sensor signals. Since the filter, when in a blocking state, stops mainly visible light, but not infrared light, the difference is essentially proportional to the amount of visible light. This reduces the influence of infrared light and corresponding unwanted blocking of the filter by infrared light sources.
US Patent Application Publication 2005/0133685 A1 shows a light shutter assembly with automatic shade level adjustment that uses a phototransistor instead of a diode in its light sensing circuit. The light shutter can be driven to one of a plurality of shade levels.
The European Standard EN 379 regulates the use of automatic welder protecting filters in Europe. It includes among others a specification for automatic welding filters that regulate the Shade Number S as a function of light intensity. Shade Numbers are defined in European Norm EN 169, and similarly in the US standard
It is thus in principle known to automatically adjust the transmission to the brightness of the welding scene. However, such products have not met with success, since fully automatic filters are only applicable in situations and welding positions where the mask's sensors correctly capture the brightness of the welding process. For other situations, a mask with a manual transmission setting has to be used. Furthermore, in a fully automatic anti-glare device, the welder's specific preference with respect to the perceived brightness is not accounted for.